Swedish University of Agricultural Sciences
Department of Soil and Environment
Evaluating solubility, aggregation and sorption
of nanosilver particles and silver ions in soils
Aidin Geranmayeh Oromieh
Master’s Thesis in Environmental Science
Soil and Water Management – Master’s Programme
Institutionen för mark och miljö, SLU
Examensarbeten 2011:08
Uppsala 2011
SLU, Swedish University of Agricultural Sciences
Faculty of Natural Resources and Agricultural Sciences
Department of Soil and Environment
Aidin Geranmayeh Oromieh
Evaluating solubility, aggregation and sorption of nanosilver particles and silver ions in soils
Supervisor: Dan Berggren Kleja, Department of Soil and Environment, SLU
Assistant supervisors: Vadim Kessler, Department of Chemistry, SLU and Sara Hallin, Department of
Microbiology, SLU
Examiner: Jan Eriksson, Department of Soil and Environment, SLU
EX0431, Independent Project in Environmental Science - Master´s thesis, 30 credits, A2E
Soil and Water Management – Master’s Programme
Institutionen för mark och miljö, SLU, Examensarbeten 2011:08
Uppsala 2011
Keywords: silver nitrate, silver nanoparticle (AgNP), soil, sorption, solubility, aggregation
Online publication: http://stud.epsilon.slu.se
Abstract
Engineered nanoparticles (ENPs) are used in so many different products. ENPs are released
into different environmental compartments. Silver nanoparticle (AgNP) is one of the most
used ENPs. AgNPs may cause damage to the environment due to their toxicity and wide
exposure. In this thesis possible exposure ways of AgNPs to the environment was reported.
Also sorption-solubility and aggregation of AgNPs and AgNO3 based on the different
concentration of Ag and pH function in clayey and sandy soil was investigated.
Results showed that sorption of both silver nanoparticles and silver ions by the soils were
increased with increasing pH. Silver (nano/ion) sorption ratio in clayey treatment was slightly
higher than sandy one. It can be due to having higher CEC value and finer texture in clay in
comparison with sand. Partitioning between nanosilver and free silver ions was investigated
by help of ultrafiltration. It can be concluded that a significant fraction of the silver
nanoparticles were oxidized and transformed to free Ag+ during the oxic experimental
conditions.
The aggregation of AgNPs and silver ions was investigated based on the different
concentration of silver in a constant pH. Aggregation of silver nanoparticulate by help of
SEM and XRD were identified in 2.5 ppm concentration of AgNPs in sandy soil. No
aggregation was found at low concentration of silver nanoparticles. No silver aggregated spot
could be recognized at 6.7, 0.65 and 0.05 ppm concentration of silver in silver nitrate polluted
soil samples.
Keywords: silver nitrate, silver nanoparticle (AgNP), soil, sorption, solubility, aggregation.
Contents
Introduction ............................................................................................................................................ 1
Nanomaterials..................................................................................................................................... 1
Release of silver nanoparticles into the environment ........................................................................ 3
Factors affecting detachment of nanoparticles from commercial products ...................................... 4
Possible fate of silver and silver nanoparticle in soils......................................................................... 4
Objective ................................................................................................................................................. 5
Materials and Methods........................................................................................................................... 5
Nanosilver ........................................................................................................................................... 5
Soil Samples ........................................................................................................................................ 5
Batch Experiment ................................................................................................................................ 5
Pilot acid-base titration experiment ............................................................................................... 5
Acid-base titration experiment with Ag+ and AgNPs ...................................................................... 6
Aggregation ......................................................................................................................................... 6
Silver size distributions in solution ..................................................................................................... 6
Results and Discussions .......................................................................................................................... 7
Characterization of nanosilver particles ............................................................................................. 7
Soil ....................................................................................................................................................... 8
Solubility of AgNPs and Ag+ as a function of pH ................................................................................. 8
Silver size distribution in the solution ............................................................................................... 11
Aggregation of silver nanoparticles in soil ........................................................................................ 12
Conclusion ............................................................................................................................................. 13
Future Study.......................................................................................................................................... 13
Acknowledgment .................................................................................................................................. 14
References ............................................................................................................................................ 15
Appendix ............................................................................................................................................... 18
Introduction
Nanomaterials
Nanotechnology is the study of materials in nanometer scale which is approximately between
1 to 100 nm [1]. It also involves how to control the formation of two and three dimensional
building blocks of molecular scale into well-defined nanostructure or nanomaterials (NMs)
[2]. Nanoparticles (NPs) exist in the environment from both natural and anthropogenic
sources. Natural NPs in air are known as ultrafine particles and in the soil and water systems
are also called colloidal particles [3]. The term ‘colloid’ refers to the particle sizes or other
suspended material in the 1nm-1µm size range. Figure 1, illustrates the most common
components of natural colloids along with their possible methods for separation and analysis.
Generally in practical work and laboratory, they can be studied by using cross-flow
ultrafiltration with nominal pore sizes between 1 nm and 0.2 to 0.45 µm [3]. In general NMs
can be categorized in different groups such as:
- Carbon based material such as Carbon nanotube (CNT) and Fullerenes
- Metal oxides: Zinc oxide (ZnO), Iron Oxide (Fe2O3), Titanium Dioxide (TiO2) NPs, etc
- Metals: Silver (Ag), Gold (Au) and Iron (Fe) NPs, etc
Figure 1. Size distributions of various types of environmental colloids and particles and analytical techniques
used for characterization. FFF=field-flow fractionation, FCS=fluorescence correlation spectroscopy,
LIBD=laser induced breakdown detection. (Picture is derived from ‘Nanomaterials in the environment:
behavior, fate, bioavailability and effects’) [3].
To study the morphology of NPs is interesting due to their unique shapes. Different shapes of
material mean different activity potential (surface energy). NMs can be formed in different
shapes such as: spheres, tubes, rods and prisms.
1
Engineered nanoparticles (ENPs) can also be classified according to their chemical
compositions and properties. NMs are synthesized by two different ways:
-
Top down strategy
Bottom up strategy
In top down strategy, well-organized assemblies directly originate from the bulk materials via
generating isolated atoms by using various distribution techniques. The majority of these
methods are physical but in the other type of methods; bottom up strategy, a molecular
component is used as a starting material which is linked with chemical reactions to form of
more complex clusters [4, 17].
Figure 2. Top down and bottom up strategy. (Picture is taken from the article ‘Manufacture nanoparticles:
An overview of their chemistry, interactions and potential environmental implications’) [4].
Having a high surface to volume ratio is a property of ENPs which makes it important for
environmental scientists to study the fate and behavior of NPs in the environment [12, 16].
This property can cause different behavior of NPs in chemical reaction in comparison with
their bulk materials. Having a high surface area per unit mass gives a specific characteristic to
the surface of NPs that makes them have a significant potent energy for reaction with other
particles.
ENMs are applied in many commercially available consumer products such as cosmetics,
textiles and paints. ENMs can reach the environment in different ways. Recent studies
showed release of nano-TiO2 from the outer layer of exterior facades through water leaching
[9]. Releasing of AgNPs from antibacterial socks to the waste water and sewage sludge
during washing process is also reported [10]. In some countries treated sewage sludge is used
as a soil amendment in order to increase the soil fertility. In case sewage sludges contain
toxic nanoparticles, they will cause toxicity to the soil bacteria. Toxic nanomaterials in
biosolids and landfills can cause toxicity for the environment. In order to know how to
control this negative effect, studying physico-chemical property of NPs like aggregation,
solubility-sorption and dispersion of NPs is crucial [8]. In ecotoxicological study,
bioavailability, bioaccumulation and aggregation of NPs are key issues to study [18].
2
Release of silver nanoparticles into the environment
ENMs may reach to environment during production of material or during incorporation of
NMs into products [5]. Mueller and Nowack (2008) reported that the current concentration of
nano-TiO2 is toxic to the aquatic organisms in lakes whereas the current concentration of
nanosilver and carbon nanotube still seems to be non hazardous but it is approaching the
toxic level [6]. They designed a model to assess the exposure of chemicals to the environment
in the absence of sufficiently detailed data. The model covers the flows of the NPs from
products to environmental comportments e.g. soil, air, water, sediment and groundwater and
also to the technical compartments such as; sewage treatment plant (STP), waste incineration
plant (WIP), landfills and recycling sites. The unit of the calculated flows is ton/year and the
modeled is stimulated based on the available data for Switzerland (Figure 3) [6].
AgNPs
Figure 3. Pathways for nano-Ag from products to the different environmental compartments, Waste
Incineration Plant (WIP), Sewage Treatment Plant (STP) and landfill are simulated by the model. Unit of the
flows is ton/year and the thickness of the arrows is proportional to the amount of silver flowing between the
compartments and dashed arrows represent the very small flows [6].
Based on the existing information and outputs from the model it is concluded that the most
dominant flows for silver nanoparticle are from production, manufacturing and consumption
to the sewage treatment plant and landfill [6].
Silver can be highly toxic to some aquatic organisms [20]. Silver has antibiotic property
which can damage the useful microbial communities in environment [8, 21]. Silver, even in
low concentration like 10 ng/l is toxic to zebra fish [22]. In some cases silver is reported to
bioaccumulate in phytoplankton and some marine invertebrates [23, 24]. Silver ions have
an inhibitory effect on bacterial growth. Ag+ tends to have a high sorption to the negatively
charged bacterial cell wall. Ag+ also leads the bacterial death due to generation of reactive
oxygen species and disruption of membrane permeability [25, 26]. Ecotoxicological study in
soil media shows that silver nanoparticles are highly toxic to soil nematodes [27]. Silver
nanoparticles with less than 5 nm diameter significantly inhibit the activity of nitrification
bacteria [12]. Toxicity of silver nanoparticles to nitrification bacteria is highly dependent on
the size of the silver nanoparticles [12]. According to [7], concentration of different
nanoparticles in sediments and sludge treated soils is increasing (figure 4).
3
Figure 4. Predicted concentrations of nanomaterials, nano-TiO2, nano-ZnO, nano-Ag, CNTs and fullerene in
sediment and sludge treated soil in U.S during the period 2001-2012 [7].
Factors affecting detachment of nanoparticles from commercial products
There are some factors which affect detachment of NPs from articles. These factors are such
as; NPs store in the object, the object’s life time, the method with which NPs are built into the
fabric and the real usage of the object [5]. Articles with an extended life time, weak
incorporation and extreme use will most likely not contain NP anymore at the time of
disposal. On the other hand, factors such as short lifetime, low usage and well built fixation
increase the likelihood that particles will not be released before disposal [13]. Exposing
articles with UV-light increases the chance of releasing NPs from the objective [14]. For
example, for silver nanoparticles, one of the most possible ways of releasing Ag+ from textile
and plastics is detachment of AgNPs in the form of silver ions [15].
Possible fate of silver and silver nanoparticle in soils
Soils are complex systems. Soil solution might contain various concentrations of organic and
inorganic ligands. Major compartments involving in metal sorption are soil organic matter
(SOM), oxide surfaces (Fe, Al, Mn…) and clay minerals [28, 29]. High pH value and CEC
enhance silver sorption to the soil due to higher negatively charged sites and more cation
exchange reactions, respectively. Soils with fine texture have higher surface areas which also
ease silver sorption [28].
Silver is sorbed more strongly to soils with high organic matter concentration than to mineral
soils. Silver can be sorbed to the soil either due to cation exchanges or complexation
reactions [28]. Soil organic matters play an important role in silver sorption and mobility
[29]. Silver strongly tends to bind with reduced sulfur groups (thiol) on soil organic matter
and form S-Ag-S bonds [30]. Regarding the silver nanoparticle in soil systems, there are not
many published papers which discuss the fate of AgNPs in soils media. But there are some
4
investigations which show the effect of different environmental factors like ionic strength,
pH, temperature, oxygen availability, time, organic matter on stability, aggregation, mobility
and silver ion release kinetics of AgNPs in aquatic medias [19, 31, 32, 33]. Recently effect of
fulvic acid on the aggregation of AgNPs in the aquatic systems was investigated and the
results surprisingly show that addition of 10 mg/l of fulvic acid had no significant affect on
the aggregation kinetic of AgNPs [34].
Objective
The objective of this thesis is to study the solubility of silver nanoparticles and silver ions in
two soil types as a function of pH and metal concentration added. Observation and
identification of nanosilver aggregated spots in polluted samples were made by help of
Scanning Electron Microscopy (SEM) and X-ray Energy Dispersion Spectrometry (XRD).
Materials and Methods
Nanosilver
Commercial silver nanoparticles (<100 nm, 99.5 % metal) were obtained from Sigma-Aldrich
Company. X-ray energy dispersion spectrometry and Scanning Electron Microscopy (SEM)
were used to characterize the silver nanoparticles. SEM and X-ray energy dispersion
spectrometry analysis have been performed in chemistry department of Swedish University
of Agricultural Science (SLU) with SEM-Hitachi TEM-1000 and Bruker X-ray diffraction
analyzer.
Soil Samples
Two different soil types were used in the study; one clayey and one sandy soil were sampled
from Ultuna and Nåntuna regions respectively in Uppsala. The texture of the soils was
analyzed by pipette and wet sieving using standard methods according to the procedure by
Ljung (1987). Water content on mass basis was measured according to international standard
ISO-11465. Carbon and Nitrogen content of soil samples based on the mass weight were
measured with LECO, CNS-1000 Elemental Analyzer. Exchangeable cations were
determined by 0.1 M BaCl2 extraction.
Batch Experiment
Pilot acid-base titration experiment
In order to investigate the buffering capacities of the soils, batch experiment with different
additions of HNO3 or Ca(OH)2 were made. 50 ml centrifuge tubes were used. 3 g of air dried
soil was added to 12 centrifuge tubes (6 tubes for sand treatment and 6 tubes for clay one),
plus various amounts of 0.1 M nitric acid (HNO3) or 0.1 M calcium hydroxide (Ca(OH)2) and
5
0.1 M calcium nitrate (Ca(NO3)2) and deionized water, see Table A1 in Appendix. Prepared
samples were shaken for two days in end-over-end shaker in room temperature 20 °C. After
shaking, the samples were centrifuged for 15 minutes at 3000 rpm in Sorvall RC 3C plus
centrifuge machine. pH was measured in 5 ml of supernatant.
Acid-base titration experiment with Ag+ and AgNPs
The pilot titration experiment resulted in a pH range in clayey soil between 4 and 7. For the
sandy soil pH varied from 3 to 6, see Table A1, Appendix. Sorption experiment with Ag+ and
AgNPs was designed based on the pilot acid-base titration to achieve the desired pH values.
Stock solution with 1.35 ppm concentration of AgNPs/AgNO3 in deionized water was
prepared in 1 liter glass bottles. Stock solution of silver nanoparticle was ultrasonicated for
10 minutes to get the homogenous suspension and delay the immediate aggregation of
nanoparticles. Immediately after ultrasonication, 15 ml of 1.35 ppm concentration of AgNPs
and AgNO3 stock solution were added to centrifuge tubes. See sample preparation schemes in
Table A2, Appendix.
After shaking and centrifuging (same instruction as for titration experiment above), pH was
measured on 5 ml of unfiltered supernatants. An aliquot of the supernatants were filtered with
0.45 µm membrane filtration. Supernatants were diluted 1000 times with 1% nitric acid in
order to make the solution ready to measure silver concentration with ICP-MS (Inductively
Coupled Plasma Mass Spectrometry, Leam 6100 ICP-MS Simplifying Ultra trace Analysis
machine). In addition dissolved organic carbon (DOC) was measured on a small proportion
of filtered (0.45 µm) supernatant with TOC 5000A Total Organic Carbon Analyzer,
Shimadzu machine. Batch experiments were designed in two replicatesof all treatments.
Aggregation
In order to study the aggregation of AgNPs and Ag+, sets of batch experiments based on the
titration experiment on sandy soil with fixed pH (4.9) and different concentration of silver
(nano/ion) was designed. See Table A1. Appendix, set 2, centrifuge tube number 11. 28.5 ml
stock solutions of silver nanoparticle with 2.5, 0.75 and 0.01 ppm concentrations and silver
nitrate with 6.7, 0.63 and 0.05 ppm concentrations were added to the centrifuge tubes
separately. The samples in centrifuge tubes were shaken for two days in end over end shaker
in room temperature 21 °C. The samples were centrifuged for 15 minutes with 3000 rpm in
Sorvall RC 3C plus centrifuge machine. Portion of supernatants were separated and sent for
silver analysis. Sedimented soils containing silver nanoparticle/silver nitrate were extracted.
Thin layers of extracted soil samples were rubbed into impermeable papers. The samples
were dried for one day in a fume cupboard. Dried soil samples were prepared for SEM and
X-ray energy dispersion investigation to find silver aggregated spots. The design of the
aggregation experiment was to study the effect of different concentrations of silver metals
(nano/ion) at a constant pH.
Silver size distributions in solution
Batch experiment was set with pH adjusted to 4 and initial constant concentration of 1.35
ppm silver ion and silver nanoparticle separately. See Table A2, Appendix tube No.1 (1CN &
1C+) and No.10 (10SN & 10S+). The batch experiments was prepared with sand and clay
6
soils with both silver nanoparticle and silver nitrate in two replicates, exactly with similar
preparation scheme to batch experiment of sorption-solubility experiment. After the batch
experiment, supernatants were filtered through 0.45 µm membrane, aliquot of supernatants
were analyzed with ICP-MS to measure the silver concentration before ultrafiltration. 10000
Dalton ultrafilters was spinned with 1%HNO3 and then rinsed with deionized water.
Ultrafilters were used immediately for filtration before letting their membrane dried. 15ml of
0.45 µm membrane filtered supernatants were added to the ultrafilters in an Amicon Ultra-15
Centrifugal Filter Device. The added supernatants in the ultrafiltration centrifuge tubes were
centrifuged for 35 minutes with 2000 rpm until most of the solution had passed the filter cap.
Silver concentrations were measured on ultrafiltered solutions in both silver nitrate and silver
nanoparticle treatments with ICP-MS.
Results and Discussions
Characterization of nanosilver particles
X-ray diffraction spectrometry shows 100% purity of silver in purchased silver
nanoparticles. The result of X-ray diffraction spectrometry is shown in figure 5.
Figure 5. X-Ray energy dispersion spectrometry result of silver nanoparticle.
Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images
of pure silver nanoparticle powder were taken. Figure 6-A and 6-B show that AgNPs have
ellipsoidal (sphere) shape.
A)
B)
Figure 6. A) SEM image of AgNPs taken at SLU.
B) TEM image of AgNPs (Sigma-Aldrich Co.).
7
Soil
Results from mechanical analysis of soil samples are shown in Table 1. Based on the
mechanical analysis, the soil texture was recognized according to USDA standard method.
The clayey Ultuna soil is clay and sandy Nåntuna soil is loamy sand. General characteristics
of soil samples are shown in Table 2.
Table 1. Mechanical analysis of the experimental soils. Particle size classes are in mm.
Clay
Fine
Medium Coarse Fine
Medium
Coarse
silt
silt
silt
sand
sand
sand
0.0020.0060.020.060.20.6d<0.002
0.006
0.02
0.06
0.2
0.6
2
8
3
1
3
12
57
17
Sand
Clay
47
12
9
10
17
Table 2. General characteristics of the experimental soils.
Clay
6.6
pH
1.8
Water content*, %
1.5
Carbon content, %
0.16
Nitrogen content, %
47
Clay, %
6
Sand, %
48
Silty, %
Clay
Soil texture
* Value is based on the mass weight.
3
3
Loss of
ignition%
5
5
Sand
5.3
0.9
2.9
0.19
8
74
29
Loamy sand
The exchangeable cations (Mg, Ca, Al, Na and K) and cations exchange capacity (CEC) in
clay and sandy soil samples are shown in (Table 3). The cation exchange capacity is about
three times higher in the clay soil compared with the sandy soil sample. This must be due to
the higher clay content, since the content of organic matter actually is lower in the clay soil as
compared to the sandy soil.
Table3. Exchangeable cations and cations exchange capacity (CEC). CEC calculated as sum of metal cations.
Exchangeable Cations (cmolc/kg)
Soil types
Mg
Ca
Al
Na
K
CEC
0.58
4.43
0.15
0.01
0.08
5.27
Sand
0.85
14.12
0.0
0.02
0.54
15.55
Clay
Solubility of AgNPs and Ag+ as a function of pH
Solubility of silver nanoparticles and silver ions was highly dependent on the pH. Results
showed that at higher pH values, both silver nanoparticles and silver ions tended to be sorbed
more strongly by the soil particles in both clay and sandy soils (Tables 4-7, Figure 7).
8
Table 4. Concentration of AgNPs & DOC in the supernatant at different pH in clay-AgNP sorption.
Acid
Base
pH
pH
DOC
DOC
Ag
Ag
addition
addition replicate 1 replicate
conc. ppm conc. ppm
conc.ppb
conc.ppb
HNO3
Ca(OH)2
2
replicate1
replicate 2
replicate 1
replicate 2
ml
ml
3
0
4.2
4.2
12.3
11.3
1.6
1.6
2.4
0
4.7
4.7
8.7
10.6
0.9
0.5
1.8
0
5.2
5.2
7.9
7.1
0.8
0.4
1.2
0
5.8
5.8
5.7
5.9
0.8
0.3
0.6
0
6.4
6.4
4.2
4.7
0.3
0.2
0
0
6.9
6.9
5.5
5.6
0.4
1.6
Table 5. Concentration of AgNO3 & DOC in the supernatant at different pH in clay-AgNO3 sorption.
Acid
Base
pH
pH
DOC
DOC
Ag
addition
addition replicate 1 replicate 2 conc. ppm
conc. ppm
conc.ppb
HNO3
Ca(OH)2
replicate 1
replicate 2
replicate 1
ml
ml
3
0
4.2
4.2
16.6
13.6
4.6
2.4
0
4.7
4.7
10.5
10.9
4.6
1.8
0
5.2
5.2
10.2
10.6
1.9
1.2
0
5.8
5.8
9.0
9.1
1.0
0.6
0
6.4
6.4
8.8
**
0.9
0
0
6.9
6.9
7.3
7.9
0.5
Table 6. Concentration of AgNPs & DOC in the supernatant at different pH in sand-AgNP sorption.
Acid
Base
pH
pH
DOC
DOC
Ag
addition
addition replicate 1 replicate 2
conc. ppm conc. ppm
conc.ppb
HNO3
Ca(OH)2
replicate 1 replicate 2
replicate 1
ml
ml
1.8
0
3.3
3.2
17.5
17.1
38.2
1.2
0
3.5
3.4
16.1
14.6
17.7
0.6
0
4.1
4.0
12.3
12.6
3.5
0
0
4.9
5.3
9.4
8.5
0.3
0
0.3
6.9
6.6
10.3
10.4
0.2
0
0.6
7.4
7.4
15.3
13.9
0.2
Table 7. Concentration of AgNO3 & DOC in the supernatant at different pH in sand-AgNO3 sorption.
Acid
Base
pH
pH
DOC
DOC
Ag
addition
addition replicate 1 replicate 2 conc. ppm
conc. ppm
conc.ppb
HNO3
Ca(OH)2
replicate 1
replicate 2
replicate 1
ml
ml
1.8
0
3.2
3.2
18.0
14.8
16.4
1.2
0
3.5
3.4
16.1
14.4
9.4
0.6
0
4.0
4.0
13.2
12.3
5.5
0
0
5.1
5.3
11.3
12.0
0.9
0
0.3
6.6
6.8
13.6
12.5
0.4
0
0.6
7.3
7.3
18.5
17.1
0.7
Ag
conc.ppb
replicate 2
1.8
0.6
1.2
0.4
0.4
0.6
Ag
conc.ppb
replicate 2
48.0
31.2
5.8
0.4
0.2
0.5
Ag
conc.ppb
replicate 2
17.9
12.9
6.4
0.5
0.5
0.9
Furthermore, silver nanoparticles and silver ions behave similarly, showing a similar pH
dependency. The solubility of silver ions is slightly higher than silver nanoparticles, but this
difference is surprisingly small (Figure 7).
9
A-1)
A-2)
Clay AgNP
14
[Ag (ppb)] & [DOC (ppm)]
[Ag (ppb)] & [DOC (ppm)]
12
10
8
Ag
R² = 0.92
6
DOC
4
R² = 0.33
2
0
0
2
4
pH
6
8
R² = 0,602
Ag
DOC
R² = 0,6626
0
10
B-1)
2
4 pH 6
8
10
B-2)
Sand AgNP
60
R² = 0,8687
[Ag (ppb)] & [DOC (ppm)]
[Ag (ppb)] & [DOC (ppm)]
Clay AgNO3
18
16
14
12
10
8
6
4
2
0
50
Ag
40
DOC
30
20
10
R² = 0,8021
0
0
2
4
pH
6
8
10
Sand AgNO3
20
18
16
14
12
10
8
6
4
2
0
R² = 0,0041
Ag
DOC
R² = 0,8136
0
2
4
pH
6
8
10
Figure 7. A-1) Solubility of AgNP and DOC vs. pH in clayey soil treatment. A-2) Solubility of AgNO3 and
DOC vs. pH in clayey soil treatment. B-1) Solubility of AgNP and DOC vs. pH in sandy soil treatment. B-2)
Solubility of AgNO3 and DOC vs. pH in sandy soil treatment.
The concentration of Ag in solutions varied from 0 to 5 ppb and from 0 to 60 ppb in clay and
sand treatment respectively. The high concentration of silver in the supernatant of sandy
treatment could be due to less free surfaces available for metal binding in sandy soil in
comparison with clayey soil. This is in accordance with the higher CEC of the clay sample
(Table 3). However, a factor also contributing to the higher concentrations of Ag (both Ag+
and AgNPs) is the fact that lower pH values were achieved in the sandy soil. Since solubility
increases with decreasing pH, this will explain, at least partly, the higher Ag concentrations
obtained in the sandy soil.
One possible explanation for the similar behavior of silver nanoparticles and silver ions could
be that they both have a net positive charge. Silver nanoparticles have zero charge [Ag(0)] in
their metallic form. However, part of the Ag(0) atoms at nanoparticle surfaces might be
ionized and oxidized when particles are exposed to aerobic environmental compartments.
10
This will give a net positive charge of the silver nanoparticles. From Figure 7, it can be
assumed that AgNPs have net positive charges in their diffuse layer since they act like silver
cations in sorption.
Silver ions might also be strongly bound by soil organic matter [28]. Silver strongly tends to
bind with reduced sulfur groups (thiol) on soil organic matter and form S-Ag-S bonds [30].
As shown in Figure 7, Ag concentrations follow DOC concentrations in suspensions of the
clay soil but not in the sandy soil. Thus, dissolution of organic matter does not seem to be the
major driver for controlling the solubility of Ag ions. Instead it is likely a pH dependent
partitioning of Ag ions between the solid and solution phases, which is governing the
solubility.
Silver size distribution in the solution
According to the manufacturer Malvern Co, it is assumed that only silver ions (Ag+) can pass
the 10000 Dalton ultrafiltration membranes [11]. Therefore by using ultrafiltration separation,
nanosilver particles and silver ions in the solution can be separated according to their size.
According to [19], silver nanoparticles can be oxidized and transformed to free silver ions in
the presence of oxygen molecules. If this occurs to a large extent with the particles introduced
into the sand and clay soil suspensions, this would be an alternative explanation for the
similar behavior of silver nanoparticles and silver ions. One way to investigate if this was the
case in my experiment was to apply ultrafiltration on equilibrium solutions.
The size distribution of silver in the solutions of silver ion and silver nanoparticle
suspensions, before and after ultrafiltration (10000 D) is shown in Figure 8.
Ag conc. (ppb) in
supernatant.
12
10
[Ag] after 0.45 µm
filtration
[Ag] after
ultrafiltration 1nm
8
6
4
2
0
Clay AgNP
Clay Ag+
Sand AgNP
Sand Ag+
Figure 8. Ag (ion/nano) concentration after and before ultrafiltration. Ag conc. in clay AgNP treatment was
below the detection limit. Error bars show the standard error values (n=2).
The concentrations of silver in the ultrafiltered solutions in clay and sand were slightly lower
than in the unfiltered (unultrafiltered) solutions (red bars are smaller than the blue bars, see
figure 8). It can therefore be concluded that a significant fraction of the added silver
nanoparticles were transformed and oxidized to the free silver ions, which could pass the
ultrafiltration membrane. In the clay AgNP treatment the silver concentrations before and
after the ultrafiltration were below the detection limit and results for this treatment was not
plotted.
11
Silver ion release from silver nanoparticles is an oxidation process which needs both
dissolved oxygen and protons [19] (Equation 1). The oxidation of silver nanoparticles is
followed by peroxide production. Peroxide is a more powerful oxidant than oxygen and
reacts faster than oxygen with AgNPs under ambient conditions [19].
Equation 1:
2Ag (s) + ½ O2 (aq) + 2H+ (aq)<----> 2Ag+ (aq) + H2O (l)
Silver ions release from silver nanoparticles may vary based on the different abiotic
environmental factors. According to [19], Silver ion release from silver nanoparticle will
increase with increasing temperature in the range of 0 – 37 °C. Silver ion release is
decreasing with increasing pH and addition of humic or fulivc acid will decrease the ion
release rate [19]. But these factors were not studied in this thesis. In this thesis kinetics of
silver ion release was investigated at constant room temperature 21 °C.
Aggregation of silver nanoparticles in soil
With help of SEM and XRD, polluted sandy soil samples with different concentration of
silver ions and silver nanoparticles were inspected carefully. An aggregated spot in treatment
with 2.5 ppm concentration of silver nanoparticles treatment was found in sandy soil
(Figure 9).
A)
B)
Figure 9. A) Aggregated spot of AgNP found in
sand treatment.
B) XRD spectrum showing the existence of Ag in
aggregated spot.
No aggregation was found in 0.01 and 0.75 ppm concentrations of silver nanoparticles. In the
treatments polluted with silver nitrate no aggregation of silver was found at any
concentration. Possible reason for having no aggregation of silver in silver nitrate treatment
could be no reduction of Ag+ to metallic silver.
Some studies in aquatic mediums show that abiotic factors such as ionic strength, pH,
concentration and salinity affect the aggregation of nanoparticles in aquatic systems [18].
NPs tend to highly interact with other molecules to reduce their surface energy. NPs bounded
12
with capping agents tend to remain stabilized [17]. In aquatic mediums, aggregation of metal
nanoparticles can be described by DLVO theory (Derjaguin, Landau, Verwey, Overbeek
theory). DLVO theory explains the forces between charge surfaces interaction in liquid
mediums. DLVO theory describes the vanderwales attraction forces and electrostatics
repulsion forces in electrical diffuse layer between two particles [18]. Soils are more complex
systems than the aquatic mediums; therefore aggregation of metal nanoparticles in soil cannot
directly be interpreted by DLVO theory. But since aggregation of silver nanoparticle could
only found in the treatment with the highest concentration of metal, it can be concluded that
concentration is a driving factor in aggregation of silver nanoparticle in soil media which also
can be interpreted by DLVO theory. The aggregation experiment in this thesis was based on
the different concentrations of silver metals (nano/ion) and did not investigate the other
abiotic factors.
Conclusion
The experiments in this work showed that the solubility-sorption of Ag+ and AgNP is
a function of pH. Ag+ and AgNP behave similarly in pH dependency. A possible explanation
for the similar behavior of Ag+and AgNP could be that, they both have the net positive
charges. In both silver ion and silver nanoparticle treatment it is shown that the solubility of
silver is higher at lower pH. Increasing the pH value, will increase the sorption of Ag. The
solubility of Ag+ is slightly higher than the AgNP but this difference is surprisingly small. By
comparing the silver sorption between the sand and clay treatments it can be understood that
clayey soil can retain more silver (nano/ion) concentration rather than sandy one. Higher
sorption of silver in clay soil can be due to its higher CEC value. DOC doesn’t seem to be
major factor for controlling the solubility of the Ag ions. Size speciation with ultrafiltration
shows that parts of the AgNPs are oxidized to Ag+ in oxic conditions. This is an alternative
explanation of similar behavior of Ag+ and AgNP. AgNPs appear to have higher aggregation
tendency than Ag+ since the aggregation spot could just be found in the treatments with silver
nanoparticles and no aggregation spot could be found in the silver ion treatments.
Future Study
Studies of the current concentration of nanoparticles in different environmental compartments
show that in close future the concentration of metal nanoparticles may reach to the toxic level
for soil microorganisms in sewage sludge treated soils, landfills and sediments. In order to
control the toxicity of NPs to the soil microorganisms, further toxicological studies of NPs is
needed. In environmental chemistry, studies of the aggregation, sorption-solubility
(dispersion) of NPs in different abiotic conditions is needed. Toxicity of NPs can be ascribed
to different shapes, charge, surface area and surface structure, wetability of NPs and also
availability of different surfactants in the solution. Also partitioning between nanoparticles
and free ions is fundamental in order to examine if the toxicity affect is due to free ions
derived from nanoparticles or intact nanoparticles or both. In literature most of the papers
13
have discussed the fate of NPs in aquatic conditions. There is not that much studies regarding
the fate of metal nanoparticles in soil.
Acknowledgment
Author acknowledges his main supervisor Prof. Dan Berggren Kleja and his co-supervisor
Prof. Vadim Kessler regarding their help and supervising of all stages of this thesis. Author is
highly thankful to his lovely parents for all of the supports in his life.
14
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17
Appendix
Table A1.
Design of pilot acid-base titration experiment on clay and sand.
Clay soil:
Tube no.
0.10 M
HNO3 ml
3
2.4
1.8
1.2
0.6
0
1
2
3
4
5
6
H2O (DW)
ml
27
27.3
27.6
27.9
28.2
28.5
0.10 M
Ca(NO3)2 ml
0
0.3
0.6
0.9
1.2
1.5
0.10 M
Ca(OH)2 ml
0
0
0
0
0
0
pH
4.2
4.7
5.2
5.8
6.4
6.9
Sand soil:
Tube no.
0.10 M
HNO3 ml
2.4
1.8
1.2
0.6
0
0
7
8
9
10
11
12
H2O (DW)
ml
27.3
27.6
27.9
28.2
28.5
28.5
0.10 M
Ca(NO3)2 ml
0.3
0.6
0.9
1.2
1.5
1.2
0.10 M
Ca(OH)2 ml
0
0
0
0
0
0.3
pH
3.1
3.2
3.5
4.0
4.9
5.9
Table A2.
Design of sorption experiment of AgNPs and AgNO3 as a function of pH in clay and sand.
Set 1)
CN: Clay AgNPs (replicate 1).
Tube no.
1 CN
2 CN
3 CN
4 CN
5 CN
6 CN
0.10 M
HNO3 ml
3
2.4
1.8
1.2
0.6
0
H2O (DW)
ml
12
12.3
12.6
12.9
13.2
13.5
0.10 M
Ca(NO3)2 ml
0
0.3
0.6
0.9
1.2
1.5
0.10 M
Ca(OH)2 ml
0
0
0
0
0
0
pH1
pH2
4.2
4.7
5.2
5.8
6.4
6.9
4.2
5.6
6.2
7.0
7.4
7.6
pH1
pH2
4.2
4.7
5.2
5.8
6.4
6.9
4.2
4.8
6.6
7.0
7.4
7.5
Add AgNPs
1.35 ppm ml
15
15
15
15
15
15
CN: Clay AgNPs (replicate 2).
Tube no.
1 CNr
2 CNr
3 CNr
4 CNr
5 CNr
6 CNr
0.10 M
HNO3 ml
3
2.4
1.8
1.2
0.6
0
H2O (DW)
ml
12
12.3
12.6
12.9
13.2
13.5
0.10 M
Ca(NO3)2 ml
0
0.3
0.6
0.9
1.2
1.5
18
0.10 M
Ca(OH)2 ml
0
0
0
0
0
0
Add AgNPs
1.35 ppm ml
15
15
15
15
15
15
C+: Clay AgNO3 (replicate 1).
Tube no.
0.10 M
HNO3 ml
3
2.4
1.8
1.2
0.6
0
1 C+
2 C+
3 C+
4 C+
5 C+
6 C+
H2O (DW)
ml
12
12.3
12.6
12.9
13.2
13.5
0.10 M
Ca(NO3)2 ml
0
0.3
0.6
0.9
1.2
1.5
0.10 M
Ca(OH)2 ml
0
0
0
0
0
0
0.10 M
Ca(NO3)2 ml
0
0.3
0.6
0.9
1.2
1.5
0.10 M
Ca(OH)2 ml
0
0
0
0
0
0
pH1
pH2
4.2
4.7
5.2
5.8
6.4
6.9
4.4
5.1
6.3
6.9
7.4
7.7
pH1
pH2
4.2
4.7
5.2
5.8
6.4
6.9
5.3
5.7
6.2
6.9
7.4
7.7
Add Ag+
1.35 ppm ml
15
15
15
15
15
15
C+: ClayAgNO3 (replicate 2).
Tube no.
0.10 M
HNO3 ml
3
2.4
1.8
1.2
0.6
0
1 C+r
2 C+r
3 C+r
4 C+r
5 C+r
6 C+r
H2O (DW)
ml
12
12.3
12.6
12.9
13.2
13.5
Add Ag+
1.35 ppm ml
15
15
15
15
15
15
Set 2)
SN: Sand AgNPs (replicate 1).
Tube
no.
8SN
9SN
10SN
11SN
12SN
13SN
0.10 M
HNO3 ml
1.8
1.2
0.6
0
0
0
H2O (DW)
ml
12.6
12.9
13.2
13.5
13.5
13.2
0.10 M
Ca(NO3)2 ml
0.6
0.9
1.2
1.5
1.2
1.2
0.10 M
Ca(OH)2 ml
0
0
0
0
0.3
0.6
pH1
pH2
3.2
3.5
4.0
4.9
5.9
***
3.3
3.5
4.1
4.9
6.9
7.4
Add AgNPs
1.35 ppm ml
15
15
15
15
15
15
SN: Sand AgNPs (replicate2).
Tube
no.
8SNr
9SNr
10SNr
11SNr
12SNr
13SNr
0.10 M
HNO3 ml
1.8
1.2
0.6
0
0
0
H2O (DW)
ml
12.6
12.9
13.2
13.5
13.5
13.2
0.10 M
Ca(NO3)2 ml
0.6
0.9
1.2
1.5
1.2
1.2
0.10 M
Ca(OH)2 ml
0
0
0
0
0.3
0.6
pH1
pH2
3.2
3.5
4.0
4.9
5.9
***
3.2
3.4
4.0
5.3
6.6
7.4
Add AgNPs
1.35 ppm ml
15
15
15
15
15
15
S+: Sand AgNO3 (replicate 1).
Tube
no.
8S+
9S+
10S+
11S+
12S+
13S+
0.10 M
HNO3 ml
1.8
1.2
0.6
0
0
0
H2O (DW)
ml
12.6
12.9
13.2
13.5
13.5
13.2
0.10 M
Ca(NO3)2 ml
0.6
0.9
1.2
1.5
1.2
1.2
0.10 M
Ca(OH)2 ml
0
0
0
0
0.3
0.6
19
pH1
pH2
3.2
3.5
4.0
4.9
5.9
***
3.2
3.5
4.0
5.1
6.6
7.3
Add Ag+
1.35 ppm ml
15
15
15
15
15
15
S+: Sand AgNO3 (replicate 2).
Tube
no.
8S+r
9S+r
10S+r
11S+r
12S+r
13S+r
0.10 M
HNO3 ml
1.8
1.2
0.6
0
0
0
H2O (DW)
ml
12.6
12.9
13.2
13.5
13.5
13.2
0.10 M
Ca(NO3)2 ml
0.6
0.9
1.2
1.5
1.2
1.2
0.10 M
Ca(OH)2 ml
0
0
0
0
0.3
0.6
pH1
pH2
3.2
3.5
4.0
4.9
5.9
***
3.2
3.4
4.0
5.3
6.8
7.3
Add Ag+
1.35 ppm ml
15
15
15
15
15
15
Note: In the appendix section, (C) stands for clay;(S) stands for sand, (N) stands for
nanoparticle, (+) stands for ion and (r) stands for replicate.
20